LibWeb: Add Page abstraction between PageView and main Frame

* A PageView is a view onto a Page object.
* A Page always has a main Frame (root of Frame tree.)
* Page has a PageClient. PageView is a PageClient.

The goal here is to allow building another kind of view onto
a Page while keeping the rest of LibWeb intact.

Libraries/LibWeb/Page.h

@@ -0,0 +1,87 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <AK/Noncopyable.h>
+#include <AK/OwnPtr.h>
+#include <AK/RefPtr.h>
+#include <LibGUI/Window.h>
+#include <LibGfx/Forward.h>
+#include <LibGfx/Palette.h>
+#include <LibWeb/Forward.h>
+
+namespace Web {
+
+class PageClient;
+
+class Page {
+    AK_MAKE_NONCOPYABLE(Page);
+    AK_MAKE_NONMOVABLE(Page);
+
+public:
+    explicit Page(PageClient&);
+    ~Page();
+
+    PageClient& client() { return m_client; }
+    const PageClient& client() const { return m_client; }
+
+    Web::Frame& main_frame() { return *m_main_frame; }
+    const Web::Frame& main_frame() const { return *m_main_frame; }
+
+    void load(const URL&);
+
+    bool handle_mouseup(const Gfx::Point&, unsigned button, unsigned modifiers);
+    bool handle_mousedown(const Gfx::Point&, unsigned button, unsigned modifiers);
+    bool handle_mousemove(const Gfx::Point&, unsigned buttons, unsigned modifiers);
+
+    Gfx::Palette palette() const;
+
+private:
+    PageClient& m_client;
+
+    RefPtr<Frame> m_main_frame;
+};
+
+class PageClient {
+public:
+    virtual void page_did_set_document_in_main_frame(Document*) { }
+    virtual void page_did_change_title(const String&) { }
+    virtual void page_did_start_loading(const URL&) { }
+    virtual void page_did_change_selection() { }
+    virtual void page_did_request_cursor_change(GUI::StandardCursor) { }
+    virtual void page_did_request_link_context_menu(const Gfx::Point&, [[maybe_unused]] const String& href, [[maybe_unused]] const String& target, [[maybe_unused]] unsigned modifiers) { }
+    virtual void page_did_click_link([[maybe_unused]] const String& href, [[maybe_unused]] const String& target, [[maybe_unused]] unsigned modifiers) { }
+    virtual void page_did_middle_click_link([[maybe_unused]] const String& href, [[maybe_unused]] const String& target, [[maybe_unused]] unsigned modifiers) { }
+    virtual void page_did_enter_tooltip_area(const Gfx::Point&, const String&) { }
+    virtual void page_did_leave_tooltip_area() { }
+    virtual void page_did_hover_link(const URL&) { }
+    virtual void page_did_unhover_link() { }
+    virtual void page_did_request_scroll_to_anchor([[maybe_unused]] const String& fragment) { }
+    virtual void page_did_invalidate(const Gfx::Rect&) { }
+};
+
+}